Extreme ultraviolet radiation, e.g., electromagnetic radiation having wavelengths of around 50 nm or less (also sometimes referred to as soft x-rays), and including radiation at a wavelength of about 13.5 nm, can be used in photolithography processes to produce extremely small features in substrates such as silicon wafers.
Methods for generating EUV radiation include converting a target material from a liquid state into a plasma state. The target material preferably includes at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. The target material can be solid, liquid or gas. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by using a laser beam to irradiate a target material having the required line-emitting element.
One LPP technique involves generating a stream of target material droplets and irradiating at least some of the droplets with laser radiation pulses. In more theoretical terms, LPP sources generate EUV radiation by depositing laser energy into a target material having at least one EUV emitting element, such as xenon (Xe), tin (Sn), or lithium (Li), creating a highly ionized plasma with electron temperatures of several 10's of eV.
The energetic radiation generated during de-excitation and recombination of these ions is emitted from the plasma in all directions. In one common arrangement, a near-normal-incidence mirror (often termed a “collector mirror” or simply a “collector”) is positioned to collect, direct, and, in some arrangements, focus the radiation to an intermediate location. The collected radiation may then be relayed from the intermediate location to a set of optics, a reticle, detectors and ultimately to a wafer.
In the EUV portion of the spectrum it is generally regarded as necessary to use reflective optics for the optical elements in the system including the collector, illuminator, and projection optics box. These reflective optics may be implemented as normal incidence optics or grazing incidence optics. At the wavelengths involved, the collector is advantageously implemented as a multi-layer mirror (“MLM”). As its name implies, this MLM is generally made up of alternating layers of material (the MLM stack) over a foundation or substrate. System optics may also be configured as a coated optical element even if it is not implemented as an MLM.
The optical element must be placed within the vacuum chamber with the plasm to collect and redirect the EUV radiation. The environment within the chamber is inimical to the optical element and so limits its useful lifetime, for example, by degrading its reflectivity. An optical element within the environment may be exposed to high energy ions or particles of target material. The particles of target material can contaminate the optical element's exposed surface. Particles of target material can also cause physical damage and localized heating of the MLM surface. The target materials may be particularly reactive with a material making up at least one layer of the optical element surface, e.g., molybdenum and silicon. Temperature stability, ion-implantation, and diffusion problems may need to be addressed even with less reactive target materials, e.g., tin, indium, or xenon. Blistering of the MLM coating must also be avoided.
There are techniques which may be employed to increase optical element lifetime despite these harsh conditions. For example, a capping layer may be placed on the optical element to protect the surface of the optical element. To make the capping layer more reflective it may also have multiple layers spaced to increase reflectivity at the wavelength of the radiation to be reflected. Such multilayer capping layers are, however, themselves prone to damage through mechanisms such as hydrogen diffusion and blistering.
In some systems H2 gas at pressures in the range of 0.5 to 3 mbar is used in the vacuum chamber for debris mitigation. In the absence of a gas, at vacuum pressure, it would be difficult if not impossible to protect the collector adequately from target material debris ejected from the plasma. Hydrogen is relatively transparent to EUV radiation having a wavelength of about 13.5 nm and so is preferred to other candidate gases such as He, Ar, or other gases which exhibit a higher absorption at about 13.5 mm.
H2 gas is introduced into the vacuum chamber to slow down the energetic debris (ions, atoms, and clusters) of target material created by the plasma. The debris is slowed down by collisions with the gas molecules. For this purpose a flow of H2 gas is used which may also be counter to the debris trajectory. This serves to reduce the damage of deposition, implantation, and sputtering target material on the optical coating of the collector. Using this method it is believed possible to slow down energetic particles with energies of several keV to a few tens of eV by the many gas collisions over the distance between the plasma site and the collector surface.
Another reason for introducing H2 gas into the vacuum chamber is to facilitate cleaning of the collector surface. The EUV radiation generated by the plasma creates hydrogen radicals H* by dissociating the H2 molecules. The hydrogen radicals H* in turn help to clean the collector surface from target material deposits on the collector surface. For example, in the case of tin as the target material, the hydrogen radicals participate in reactions on the collector surface that lead to the formation of volatile gaseous stannane (SnH4) which can be pumped away. For this chemical path to be efficient it is preferred that there is a low H recombination rate (to form back into H2 molecules) on the collector surface so that the hydrogen radicals are available instead for attaching to the Sn to form SnH4. Generally, a surface consisting of non-metallic compounds such as nitrides, carbides, borides and oxides has a lower H recombination rate as compared to a surface consisting of pure metals.
The use of H2 gas, however, can have a negative effect on a coating applied to the collector caused by both the light hydrogen atoms and molecules on the coating. It is believed that the hydrogen atoms are so small that they can easily diffuse several layers deep into a collector configured as a multilayer mirror. Low energy hydrogen can also be implanted near the surface and can diffuse into the collector cap and layers of the multilayer mirror beneath the cap. These phenomena most severely affect outermost layers (e.g., the first 1 μm).
Once atomic hydrogen invades the body of the multilayer mirror it can bond to Si, get trapped at layer boundaries and interfaces, or both. Hydrogen can diffuse through the MLM stack to the bonding layer below and even to the substrate. The magnitude of these effects depends on the fluence of hydrogen to the surface, the hydrogen dose absorbed, and the concentration of hydrogen in these regions. If the hydrogen concentration is above a certain threshold it can form bubbles of gaseous hydrogen compounds, either recombining to H2 molecules or perhaps also forming hydrides. This can happen most severely typically underneath the MLM stack or in the substrate layer. When a gas bubble starts to form there is a high probability that it will grow in the presence of additional hydrogen. If such bubbles do form then their internal gas pressure will deform the layer above the bubble, leading to the formation of blisters on the coating of various sizes. The layer may then burst, thus releasing the gas below and material above this, area, resulting in delamination of the coating.
A blistered coating creates several problems. It has a higher surface area and is more prone to degradation by oxidation and other contaminants and by deposition of target material. Due to higher absorption this generally leads to a reduction of EUV reflectance. A blistered coating also scatters more light due to higher roughness and thus leads to significantly reduced EUV reflectance at desired angles, even though the undamaged layers below still contribute to reflection of EUV light and even if the target material deposits are removed by cleaning. Blisters also cause a change in reflection of out-of-band (OoB) light which can include light generated by the plasma as well as light from the drive laser and the loss of effectiveness of elements such as gratings used to deliberately scatter light from the drive laser.
Collector blistering due to H* exposure severely limits lifetime and greatly impacts system availability. A current model, for damage is H* adsorption at the surface where defects are present and transport of H atoms to regions within the bulk. A primary indication is that this occurs below the MLM stack in the adherence layer. H* generation scales with EUV power so the problem of EUV MLM optical element blistering can be expected to grow worse as the power of the source increases.
In addition to these effects, hydrogen uptake and penetration can also lead to embrittlement of metal layers and thus cause layer degradation.
There thus is a need to exploit the advantages with respect to enhancing the EUV reflectance of using a multilayer optic while at the same time having an optic that is resistant to hydrogen damage such as blistering.